Forced cooling means and method for continuous strip furnaces



Dec. 18, 1962 A. H. VAUGHAN ETA 3,068,586

FORCED COOLING MEANS AND METHOD FOR CONTINUOUS STRIP FURNACES Filed Feb. 18, 1959 4 Sheets-Sheet 1 INVENTORS ArthzzrIiV luau,

Elmer L. '5;

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18, 1962 A. H. VAUGHAN EI'AL 3,

FORCED COOLING MEANS AND METHOD FOR CONTINUOUS STRIP FURNACES Filed Feb. 18, 1959 4 Sheets-Sheet 5 INVENTORS ArfilzwrH. Vazqg'kwn 1;, By ElmerLJrn' lz ATTORNEYS atent Gfilice 3,068,586 Patented Dec. 18, 1962 3,068,586 FORCED COOLING MEANS AND METHOD FOR CONTINUOUS STRIP FURNACES Arthur H. Vaughan, Salem, and Elmer L. Kerr, Damascus, Ohio, assignors to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Filed Feb. 18, 1959, Ser. No. 794,139 6 Claims. (Cl. 34-20) The invention relates to cooling means for continuous strip furnaces, and more particularly to such means using forced convection cooling in both horizontal and vertical type furnaces for annealing or other thermal treatment of strip metal.

It is common practice in thermal treatment of strip metalpsuch as the annealing of carbon or silicon steel strip, to uncoil the strip and pass it continuously and successively through heating, soaking and cooling chamhers, and then recoil it.

Generally, a non-oxidizing atmosphere is maintained in all of the chambers, in order to protect the strip surface until it is delivered into the air at a safe temperature. This temperature may be about 250 F. in the case of carbon steel, if avoidance of oxidation is the only criterion.

However, temperatures as low as 150 F. may be re quired in some cases, while in other cases, where a small amount of oxidation is permissible or desired, higher temperatures up to 450 F. may be selected.

Under present practice, such furnaces utilizing natural convection are of very great length, requiring considerable floor space for horizontal-type furnaces or great height for vertical furnaces, and being quite expensive to construct.

Many attempts have been made to accomplish effective cooling by forced convection, but no completely satisfactory and efiicient means has been produced therefor. Gas flow generally parallel to the strip surface has been used in most cases.

For instance, in one case using parallel flow, the gases moving in the work passage make contact both with the work and with water-cooled surfaces. However, it has been found impractical to use suflicient gas velocity to produce heat transfer coefiicients as high as are desired or necessary.

The present invention contemplates the provision of forced convection cooling in order to overcome the disadvantages of present practice.

The general object of the invention is to provide improved means for rapid forced cooling of metal strip, with which a controlled atmosphere can be used, and in which higher rates of heat transfer can be realized than in systems heretofore available.

The value of accelerated cooling is shown by an example. Consider a furnace for the annealing of low carbon steel strip, such as tin plate, with a capacity of 30,000 lb. per hour, 30 wide x .010" thick. The work is to be heated to 1350 F., soaked at temperature 8 seconds, cooled to 900 F. in 20 seconds, then cooled at any practicable rate to 250 F. The effective emissivity of the strip surface is .25 x perfect black-body.

The strip speed at .010" thickness, 30,000 lb. per hour, is about 500 feet per minute or 8.33 feet per second. Thus the zone lengths for the specified soaking and retarded coolingtimes are 66.7 and 167 feet respectively. Calculation-s show that heating time in a 1600 F. chamber is about 23 seconds, requiring 192 feet length.

The final cooling from 900 F. to 250 F. takes place by a combination of radiation and natural convection. It is common practice to lead the strip through a passage or a series of passages having water-cooled walls. Radiation takes place at emissivity of .25 (or possibly somewhat lower because of the low work temperatures in this stage). Experience shows that the overall natural convection coetlicient, based on the temperature difference w-ork-to-cooling water, seldom exceeds 1.75 B.t.u. per (sq. ft., hr., E). On this basis, the required time from 900 F. to 250 F., with cooling water temperature of F. is about 54 seconds. The resulting length is 452 feet. This is 2.35 times the heating length, which is a typical ratio for natural convection.

If forced convection can be used with sufi'icient effectiveness to realize a coefficient of 6 instead of the natural convection value of .75, the required time becomes about 17 seconds and required length 142 feet. These are 31.5% of the natural convection requirements.

The advantage of forced cooling means, having this degree of effectiveness, is obvious. With the present invention, an overall coeflicient of 6 is readily obtainable. Even though the cost per foot of length for forced convection, is greater than for natural convection, the re duction of more than two-thirds in the length results in a net saving as well as a smaller space requirement.

We have found by experiment that if properly disposed jets of gas are caused to impinge upon the strip surfaces, very high average rates of heat transfer can beobtained. For a cooling chamber profile having a clearance of say eight inches from each face of the strip, and a width of about twelve inches greater than the strip, a given transfer coefficient can 'be secured by impinging-jet cooling with much less total fan capacity and fan cost than with parallel flow.

It is therefore another object of the invention to provide forced convection cooling means for continuous strip furnaces, utilizing the principle of impinging-jet cooling of the strip.

The above and other objects, apparent from the draw ings and following description, may be attained, the above described difliculties overcome and the advantages and results obtained, by the apparatus, construction, arrangement and combinations, subcombinations and parts which comprise the present invention, a preferred embodiment of which, illustrative of the best mode in which applicants have contemplated applying the principle, being set forth in detail in the following description and illustrated in the accompanying drawings.

Reference is now made to the accompanying drawings, showing preferred embodiments of the invention, in which:

FIGS. la and 1b show a complete side elevation of a strip annealing furnace of the single-pass horizontaltype embodying the invention, having successive zones for heatin soaking, retarded cooling, and for final forced cooling in accordance with the invention, showing uncoiling, shearing, welding and looping equipment of usual type at the feed end, and looping, shearing and recoiling equipment at the discharge end of the furnace;

FIG. 2 is a longitudinal elevation, on a larger scale, of one section of the forced cooling zone of a horizontaltype furnace;

FIG. 3 is a transverse sectional view, taken on the line 33, FIG. 2;

FIG. 4 is an elevation of a multi-pass vertical-type furnace embodying the invention, such as is often used for large capacities with thin strip, not showing the terminal equipment therefor which would be generally of the type shown on the horizontal-type furnace;

FIG. 5 is an enlarged elevation of the complete forced cooling zone (six vertical strip passes) for the verticaltype furnace shown in FIG. 4;

FIG. 6 is a vertical sectional view taken on the line 6 6, FIG. 5;

FIG. 7 is a transverse sectional view taken on the line 7-7, FIG.

FIG. 8 is a diagrammatic, vertical, longitudinal section of the forced cooling zone for horizontal-type fur-. pace, as shown in FIGS. 2 and 3, showing the gas flow .as it would presumably occur in the region of the jets;

FIG. 9 is a view similar to FIG. 8, showing a slight modification in which nipples are substituted for the orifices in the plenum plates;

FIG. 10 is a fragmentary perspective view of a portion of one of the plenum plates, showing the orifices therein in the form of an interrupted transverse slot;

FIG. 11 is a fragmentary perspective view of a portion of one of the plenum plates showing the interrupted slot located in an elevated portion'or hollow rib; and,

FIG. 12 is a fragmentary perspective showing one of the nipples of FIG. 9.

Referring first to the horizontal-type furnace shown in FIGS. 1a, 1b, 2 and 3, in which similar numerals refer to similar parts throughout, the equipment shown at the feed end of the furnace comprises an uncoiler 15, bridle rolls 16, a shear 17, a welder 18, and a looping tower 19.

The horizontal furnace comprises the heating zone, indicated generally at 20, with entrance seal rolls 21 at the entrance end thereof, and driven support rolls 22, suitably spaced throughout the length of the heating zone, to support the continuously moving strip 23.

Directly beyond and communicating with the discharge end of the heating zone 2- is the soaking zone, indicated generally at 24, having the spaced support rolls 25 therein for supporting the strip 23. The retarded cooling zone, indicating generally at 26, is located directly beyond and communicates with the discharge end of the soaking z'one 2-4 and is also provided with spaced support rolls 27 for the strip 23.

The forced cooling zone, indicated generally at 28, is located directly beyond and communicates with the discharge end of the retarded cooling zone 26 and is made up of a plurality of similar sections, as indicated at 29. Any desired number of these sections may be provided, very satisfactory results being obtainable, for a capacity such as that of the example ther'etofore given, with the use of seven such sections in the forced cooling zone.

An exit vestibule 30 is located at the discharge end of the forced cooling zone 28 and provided with exit sealing rolls 31. As the strip emerges from the forced cooling zone it passes through the looping tower 32, then through the bridle rolls 33 and shear 34 to the recoiler 35.

It should be understood that the equipment at the entrance and discharge ends of the furnace may be of any usual and well known design. Also, the heating zone 20, soaking zone 24 and retarded cooling zone 26 may all be of conventional design, the invention residing particularly in the forced cooling zone indicated generally at 28 and comprising the similar sections as indicated at 29. v

The detail construction of one of the similar sections 29 of the forced cooling 'zone'apparatus is shown on a larger scale in FIGS..2 and 3. Each of these sections 29 includes a steel casing 36 through which the strip 23 is continuously passed longitudinally, being supported at intervals therein upon the driven rollers 37 located at suitably spaced intervals therein and driven in any usual and well known manner, as by a sprocket chain (not shown) engaging the sprocket wheels 38 upon the roll shafts, or axles 39, which are journalled in bearings 40 upon opposite sides of the casing 36.

A plenum chamber 41 is located above the path of the strip, being enclosed by the top plate 42 of the casing 36, the vertical plates 43, horizontal bottom plate 44 and end flanges 45. A similar plenum chamber 46 'is located below the strip and enclosed by the casing bottom plate 47, vertical plates 48, horizontal plate 49 and end flanges 45.

As shown in FIGS. 2 and 3, the plates 44 and 49 are perforated, usually with round orifices or holes 50, of a diameter not less than about one-eighth of the distance between the plate 44 or 49 and the plane of the strip 23.

The area of the orifices 50 should be about 2% to 4% of the gross plate area. These orifices may be advantageously disposed at intervals in rows at right angles to the direction of strip travel.

The plenum chambers 41 and 46 have widths slightly more than the maxium strip width plus any probable deviation in position the strip may assume as it is moved through the forced cooling apparatus. Thus, the orifices 50 are preferably so located that jets therefrom will act upon the full width of any strip passing through the apparatus.

Jets issuing from the orifices 50 in the plates 44 and 49 impinge upon the upper and lower surfaces respectively of the strip. After the jet identity has been lost, the gas travels transversely between rows of jets to the spaces 51 which extend longitudinally within the casing and terminate at the end flanges 45.

The gas then flows upward through the openings 52 in the top plate 42 of the main casing 36, into the chamber 53, then through a bank of finned cooling coils 54 to another chamber 55.

An involute-type fan housing 56 is located in this chamber with inlet 57 and discharge 58. The centrifugal fan 59 is mounted within the fan housing 56 and adapted to be rotated by any usual and well known power means.

The discharge 58 from the centrifugal fan communicates with the duct 60 leading to the lower plenum chamber 46 and the duct 61 leading to the upper plenum chamber 41. Thus, a closed gas circuit is thereby provided through which the gas within the casing may be recirculated by operation of the centrifugal fan 59.

As shown in FIG. 2, the fan may be driven by a gastight motor 62. Motor 62, mounting plate 63 and fan 59 may be removed when necessary for servicing. It will be obvious that the complete cooling chamber section should be gas-tight for controlled-atmosphere operation.

For this purpose, adjoining sections 29 are connected together by the flanges 45, which may be bolted and gasketed, or weldedas desired. The openings 65 in the flanges 45, for passage of the strip 23, are only as large as necessary to allow for tracking variations of the strip, and movement of threading devices carried through by the rollers 37 when placing the equipment in operation.

The side edges of the openings 65 may be as shown at 66 in FIG. 3, the top edge at 67 and the bottom edge at 68. Thus, the several sections 29 of the cooling zone apparatus are substantially isolated from one another, and since in operation the pressure difference beween the entrance and exit end of a section is very small, the net volume of longitudinal transfer between sections is very small.

In the operation of the forced cooling apparatus, the gas leaving the cooling coil bank 54 is at the lowest temperature of any point in the circuit. A slight temperature rise, corresponding to the horsepower output of the motor, occurs in the fan, and the gas then enters the upper and lower plenum chambers 41 and 46.

The jets of gas forced through the orifices 50 in the plates 44 and 49 of the upper and lower plenum chambers respectively impinge upon the upper and lower surfaces of the moving strip 23, and the cooling gas extracts heat therefrom.

The gas also extracts heat from the plates 44 and 49 and other surfaces to which heat has been transferred by .direct radiation from the moving strip 23. The gas temperature at the entering side of the cooling coil bank 54 is the highest in the circuit and heat is now removed from the gas by contact with the cooling coils.

In the example heretofore given, the strip is cooled from 900 to 250 F. If this is done in the seven cooling sections 29 of the forced cooling zone, a first approximation of the average strip temperatures in the successive sections is 825 F., 680 F., 555 F., 458 F., 380 F., 318 F., 270 F.

If the mean water temperature in the coils 54 is 100 F., the overall average difference work-to-water in each section is work temperature minus 100 F. Thus, obviously the gas temperature will be at some point between the work and water temperatures.

In actual practice, sufficient cooling coil surface is provided to produce a difference between work and cooled gas temperatures which is at least two-thirds of the total temperature difference, work-to-water. The greater this ratio, the more heat can be extracted from the work There are two reasons for this: first, the difference is greater between work and gas, and second, the gas is denser, which increases the transfer cofiicient from work to gas. Laboratory tests show that a transfer coefficient at the strip surface of at least 12 B.t.u. per square foot, hour, F., can readily be realized with gas at 70 F., having thermal properties equivalent to air.

densities, assuming a velocity such as would give a coefiicient h of 12.0 at 70 F.

For the first section 29, this temperature will be about 825 F.2/3 (825100) or 342 F. At this temperature the density is 530/802 or 66% of standard. Coefficient h will be 12 .66- or 8.63 approximately. For the seventh section 29, gas temperature will be 270-2/3 (270-100) or 157 F. The corresponding value of density is .86 standard, and h==12 .86- or about 10.65. The mean for all seven sections 29 is at least 9.6 B.t.u. per square foot, hour, F. at the strip.

We may use an overall coefiicient U equal to 2/3 9.6 or 6.4 with the total difference work-to-water, which gives the same result and avoids the necessity of evaluating gas temperatures. This value of overall coefficient approximates the value of 6 used in the earlier discussion.

We have so far found that a velocity giving about this value U=6, with gases approximately equivalent to air, is a good economic choice of furnace work. However, considerably higher values are practicable. Also, furnace atmosphere mixtures containing hydrogen will give higher values because of increased thermal conductivity. We estimate that dissociated ammonia, 75% hydrogen and nitrogen, which is often used, may increase the value of U to about 1.75 times that for air at a given velocity.

FIG. 8 shows diagrammatically the gas fiow as it may be presumed to occur in the region of the jets or orifices 50 of FIGS. 2 and 3. The orifices 50 may be located in a square pattern, with transverse center lines at 70, and on longitudinal center lines such as shown in FIG. 3. If desired, the spacing of transverse and longitudinal center lines may be, but is not necessarily, equal.

Jets of gas issue from the orifices 50, and assuming a proper ratio of diameter to distance, impinge at 71 upon the strip 23 while still well defined and at high velocity. Heat transfer between strip and gas presumably occurs at a very high local rate in the immediate area of impingement.

The gas now flows radially, as at 72, from this region in a thin fiat stream, with considerable turbulence and encounters similar flat streams from adjacent jets, as at 73. During the radial flow, effective heat transfer is still maintained because the velocity of gas remains high and contact with the strip is intimate.

After the radial flows from adjoining orifices have met,

a general flow takes place away from the strip and into 5 is between rows of jets toward the strip edges. For this reason, we locate collecting passages 51 adjacent the strip edges as shown in FIG. 3.

The orifices 50 are not necessarily located in a square pattern. Those in adjoining transverse rows, on center lines 70, may be staggered. This has the advantage of more complete coverage of the strip surfaces. However, it is important to have well defined and easy escape lanes, such as provided by spaces 74, leading to the collecting passages. Rather than orifices, whose discharge coefficient is about .60, we may use nozzles with a coefiic'ient of about .90. Thus a higher velocity can be attained with the same power expenditures.

In some cases it may be advantageous to provide large escape spaces while yet maintaining a short distance between orifices, nozzles or slots and the work. This is likely in case of a wide strip, where the volume of gas from spent jets may be large by the time the strip edge is reached. In this case, a plurality of nipples 75, as shown diagrammatically in FIG. 9, may be attached at one open end to the plenum plates 44 and 49, with orifices 76 at their other ends. Thus the spaces 74 are enlarged and more area provided for the exit of spent gas. i

In FIG. 10 is shown a slight modification, in which either or both of the plenum plates 44 and 49 may be provided with an interrupted transverse slot 77 formed directly in the plenum plate, instead of the orifices 50.

As a further modification, as shown in FIG. 11, interrupted transverse slots 77 may be displaced forwardly toward the strip 23 by virtue of elevated portions or hollow transverse ribs 78 formed in the plenum plate.

It should be understood that the only reason for interrupting the slots 77 and 77' of FIGS. 10 and 11 is that this is a relatively simple and harmless way to provide bridges 77a to tie the edges of the slot together and prevent them from buckling out of position, which might produce a ragged or improperly directed jet.

It will be obvious that the edges of the slot could be tied together by means of their bridges connected at opposite ends to the edges of the slot, and located within the plenum chamber at a suitable distanct from the slot so as not actually to interrupt the jet.

Although we have described the most effective arrangement, in which orifices or jets are disposed in rows at right angles to the length of the strip, or slots are placed in like relation, it will be obvious that some departure from the geometric arrangement may be made without materially increasing the interference by flow of gas from spent jets.

Reference is now made to the embodiment of the invention illustrated in FIGS. 4 to 7, showing a multi-pass vertical-type furnace. In this type of furnace there is not enough room between strip passes for the fans and cooling coils, such as are used in the horizontal-type furnace, as shown in FIGS. 2 and 3. However. it is possible to apply the same principles, employing plenum chambers, gas exit passages adjacent and parallel to the strip edges, with fans and cooling coils disposed outwardly beyond the exit passages.

In this vertical-type furnace, the forced cooling chamber assembly is shown as comprising three similar sections indicated at 80, 80a and 80b, within each of which two vertical strip passes are located. The strip enters at the lower left of the section 80, as at 81, passing around the lower roll 82, then up through the section 80 as indicated at 85, around the top roll 84 and down through section 80 as shown at 85.

The strip passes out of the lower end of section 80, around the lower roll 86, then up through section 80a, as shown at 87, around the upper roll 88 and down through section 80a, as indicated at 89, passing out of the lower end of section 80a and around'the lower roll 90.

Then the strip passes up through the section 8011, as indicated at 91, around the upper roll 92., and then down,

7 as shown at 93, being finally discharged from the cooling chamber through the sealing rolls 94.

Each of the chambers 80, 80a and 80b of the forced cooling zone is of the same general construction. Extending vertically through the active height of each section, but slightly less than the clear distance between the upper and lower rolls, are a full-depth plenum chamber 95 and two half-depth plenum chambers 96.

The full-depth plenum chambers 95 are centrally located in each of the sections 36, 39a and 8th), and the half-depth plenum chambers 96 are located at opposite sides of the sections, whereby the strip passes upwardly through each section between the central plenum fulldepth chamber 95 and one of the half-depth plenum V chambers 96, and downwardly between the central fulldepth plenum chamber and the other half-depth plenum chamber 96. a

Each central full-depth plenum chamber 95 is enclosed by side walls 97, edge walls 98 and upper and lower end walls 99 and 100 respectively. Each half-depth plenum chamber is enclosed on one side by the adjacent side wall 101 of the corresponding section 80, 860 or 83b of the forced cooling assembly, on the other side by the side wall 102, at opposite edges by the edge walls 103, and at the upper and lower ends by the top and bottom walls 1th!- and 195 respectively.

The side walls 97 and 19-2 of the plenum chambers 95 and 96 respectively, facing the strip are perforated as at 106, in the manner of the plates 44 and 49 of the plenum chambers in the horizontal-type furnace, as above illustrated and described in detail, so as to impinge jets of cooling gas upon both surfaces of the moving strip.

Exit passages 16? are disposed vertically, adjacent and parallel to the strip edges, and the plenum chambers 95 and 96 and exit passages 197 are preferably divided into upper and lower portions by the horizontal, transverse plates 108.

Centrifugal fans 109. are located within each side of the housing 110 and have inlets 111 communicating with the exit passages 13'], and outlets 112 delivering through ducts 113 to the cooling coils 114, and thence by ducts 115, fulldepth ducts 116 and half-depth ducts 117 to the plenum chambers 95 and 96 respectively.

There are thus two closed circuits in each section of the forced cooling assembly, through which the furnace atmosphere may be circulated. Although similar fans, coils and ducts are shown on each side of each section, in FIG. 6, it is pointed out that it would be possible, without departing from the principle of the invention, to locate these fans, cooling coils and ducts on one side only of each section.

Heat transfer coefficients can be realized in'this construction which are similar to those previously given. In this case, because two strip passes rather than one are served by a common gas circuit, it may be economical to use lower gas temperatures so that a difference between strip and gas may remain until the strip leaves the section.

A heat transfer coefficient of 12 B.t.u. per square foot, hour, F. from strip to air can be obtained with air temperature of 70 F. and with an average velocity of about 45 f.p.rn. normal to the strip plane per surface, the jets being produced by punched orifices in the plenum chamber plates of diameter equal to about one-sixth the distance from plates to strip, and an orifice area about 2 percent of the gross area. This corresponds to a jet velocity of 2250 f.p.m. based upon hole area, 3750 f.p.-m. at the vena contracta area, and a pressure drop of about .88" water column across the orifices.

In FIG. 4 is shown a complete vertical-type, multipass strip furnace provided with the vertical, multi-pass forced cooling assembly shown in FIGS. 5, 6 and 7, and above described in detail.

This vertical multi-pass strip furnace comprises a substantially rectangular housing 120, which may be of the approximate height of the forced cooling assembly 80, a and 80b. Vertical partition walls 121 and 122, extending from the top of the housing to points near the bottom thereof, divide said housing into the heating zone 123, soaking zone 124 and retarded cooling zone 125.

Uncoiling, shearing, welding and looping equipment of the general type shown in FIG. la, may be located in advance of the vertical furnace. The multiple strips 23a pass from this equipment at the feed end of the furnace, around the roll 126 and upward through the entrance sealing rolls 127 into the heating zone 123 of the furnace.

Upper and lower rolls 128 and 129 respectively are located in the heating zone 123, soaking zone 124 and retarded cooling zone 125, the multiple strips passing alternately up and down over said rolls through all three of said zones, and then to the forced cooling assembly.

After passing through the exit sealing rolls 94 of the forced cooling assembly, the multiple strips pass around the roll 13%, and thence to terminal equipment of the general character shown in FIG. 1b.

In the foregoing disclosure it will be apparent that in both the horizontal and vertical-type furnaces the flow of gas from spent jets is not required to travel among other jets for a distance greater than half the strip width until it passes into one of the collecting ducts located at each longitudinal edge.

It should be understood, however, that a collecting duct may be located at. only one longitudinal edge, in which case the spent gas would need to travel among other jets for a distance greater than the width of the strip. It has been found that longer travel of spent gas among other jets is undesirable.

In the foregoing description, certain terms have been used for brevity, clearness and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are used for descriptive purposes herein and are intended to be broadly construed.

Moreover, the embodiments of the improved construction illustrated and described herein are by way of example, and the scope of the present invention is not limited to the exact details of construction.

Having now described the invention or discovery, the construction, the operation, and use of preferred embodiments thereof, and the advantageous new and useful results obtained thereby; the new and useful construction, and reasonable mechanical equivalents thereof obvious to those skilled in the art, are setforth in the appended claims.

We claim:

1. In apparatus for forced cooling of a strip, walls enclosing an elongated main chamber, means forming a pair of oppositely disposed plenum chambers in said main chamber and including a front wall for each plenum chamber, said front walls facing and being spaced from each other to form an elongated work passage therebetween, means for moving a strip longitudinally through said work passage, jet means associated with each of said front walls for directing a plurality of spaced jets of gaseous fluid against the flat surfaces of said strip, th area of the jet means being 2% to 4% of the gross front wall area, and the diameter of said jet means being at least one-eighth of the distance between the front wall and the plane of the strip, a collecting duct adjacent at least one longitudinal edge of said work passage, means for cooling the gaseous fluid, and means for moving gaseous fluid in a closed circuit from said plenum chamb rs through said jet means, thence to said collecting duct, thence through said cooling means, and thence under pres sure to said plenum chambers, said cooling means having sufiicient cooling surface for maintaining a mean temperature difference between the strip and the cooled gaseous fluid which is at least two-thirds of the overall temperature difference between the strip and the cooling means.

2. In apparatus for forced cooling of a strip, walls e closing an elongated main chamber, means forming a pair of oppositely disposed plenum chambers in said mai chamber and including a front wall for each plenum chamber, said front walls facing and being spaced from each other to form an elongated work passage therebetween, jet means associated with each of said front Walls for directing a plurality of spaced jets of gaseous fluid against the flat surfaces of said strip, the area of the jet means being 2% to 4% of the gross front wall area, and the diameter of said jet means being at least one-eighth of the distance between the front wall and the plane of the strip, a collecting duct adjacent at least one longitudinal edge of said work passage, means for cooling the gaseous fluid, and means for moving gaseous fluid in a closed circuit from said plenum chambers through said jet means, thence to said collecting duct, thence through said cooling means, and thence under pressure to said plenum chambers.

3. Apparatus for forced cooling of a strip, a series of means defining a plurality of identical cooling chamber sections located end to end, means forming a pair of oppositely disposed plenum chambers in each cooling chamber section and including a front wall for each plenum chamber, said front walls facing and being spaced from each other to form an elongated work passage therebetween, means for moving a strip longitudinally through said work passages, jet means associated with each of said front walls for directing a plurality of spaced jets of gaseous fluid against opposite surfaces of said strip, the area of the jet means being 2% to 4% of the gross front wall area, and the diameter of said jet means being at least one-eighth of the distance between the front wall and the plane of the strip, a collecting duct adjacent at least one longitudinal edge of each work passage, heat transfer means in each cooling chamber section for cooling the gaseous fluid by transferring heat to a coolant fluid, means for moving gaseous fluid in a closed circuit from said plenum chamber through said jet means, thence to said collecting duct, thence through said heat transfer means, and thence under pressure to said plenum chambers in each cooling chamber, the flow of gaseous fluid in each section being isolated from the external atmosphere and separate from the flow in adjoining sections, said heat transfer means having suflicient cooling surface for maintaining in each section a mean temperature dilference between the strip and the cooled gaseous fluid which iS at least two-thirds of the mean overall temperature difference between the strip and the coolant fluid.

4. Apparatus for forced cooling of a strip as defined in claim 3, in which the cooling means comprises heat exchanger means through which the gaseous fluid and a coolant fluid are passed, the heat exchanger means in each cooling chamber section having substantially eq effective heat exchange surfaces.

5. Apparatus for forced cooling of a strip as defined in claim 3, in which substantially equal volume flow of gaseous fluid is maintained in each cooling chamber section.

6. A method for forced cooling of strip including the steps of progressively moving the strip longitudinally through a series of cooling chambers, directing gaseous fluid in a plurality of high velocity jets upon opposite surfaces of the strip in each cooling chamber, collecting the gaseous fluid after contact with the strip in each cooling chamber, fluid cooling the gaseous fluid collecting in each cooling chamber by transferring heat to a coolant fluid in heat exchange means, and again directing the cooled gaseous fluid in high velocity jets upon the surfaces of the strip in each cooling chamber while maintaining a mean difierence between the strip and the cooled gas temperature in each chamber which is at least twothirds of the mean total difference from the strip to the coolant fluid, the mean strip temperature in successive cooling chambers being in the order of 825 F., 680 F., 555 F., 458 F., 380 F., 318 F. and 270 F References Cited in the file of this patent UNITED STATES PATENTS 2,265,071 Hartenbach Dec. 2, 1941 2,422,105 Lehrer June 10, 1947 2,483,605 Abramson Oct. 4, 1949 2,534,973 -Ipsen et al Dec. 19, 1950 2,591,621 Shedga Apr. 1, 1952 2,693,353 Vaughan Nov. 2, 1954 2,696,055 Murphey Dec. 7, 1954 2,772,486 Johanson Dec. 4, 1956 2,876,555 Vander Pyl Mar. 10, 1959 2,896,335 Dungler July 28, 1959 

